Quench system

ABSTRACT

Low-carbon unalloyed steel strip is heated to a temperature at least above the A1 point, preferably above the A3 point, and is quenched to transform substantially all the austenite to martensite. Quenching is accomplished by passing the heated strip through an elongated restricted quench channel having highvelocity quench liquid flowing through the channel, either concurrently or countercurrently. The temperature and flow velocity of the quench liquid are regulated in the channel so as to provide an initial high rate of heat withdrawal from the strip and a subsequent lower rate of heat withdrawal during the time the strip is quenched through the temperature range of martensite formation, thereby effecting tempering of the martensite.

United States Patent [72] Inventor Harold L. Taylor 3,410,734 11/1968Taylor 148/143 Hammond Ind.

Primary Exammer- Richard 0. Dean [21] Appl. No. 819,756 n Filed p 1969Attorney Hibben, Noyes & ickne [45] Patented Oct. 26, 1971 [73] AssigneeInland Steel Company Chicago, 11].

ABSTRACT: Low-carbon unalloyed steel strip is heated to a [54] QUENCHSYSTEM temperature at least above the A, point, preferably above the 19Claims, 2 Drawing FigS A point, and 1S quenched to transformsubstantially all the austemte to martensite. Quenching 1S accomplishedby passing U-S. the heated tri through an elongated restricted quenchhan- 148/361 148/153, 148/156 148/157 nel having high-velocity quenchliquid flowing through the Ila. channel either on urrently orcoun[ercurren[ly The tem- [50] Field of Search 148/143, perature and flvelocity f the quench liquid are regulated 1451 361 1551 157 in thechannel so as to provide an initial high rate of heat 56 R f Ct dwithdrawal from the strip and a subsequent lower rate of heat 1 eerences l e withdrawal during the time the strip is quenched through theUNITED STATES PATENTS temperature range of martensite formation, therebyeffecting 3,378,360 4/1968 McFarland 148/36 tempering ofthe martensite.

4 36 v C v 37 I l 4 QUENCH SYSTEM This invention relates to a novelmethod for continuously quenching metal strip and more particularly forquenching low-carbon unalloyed steel strip to obtain a microstructurewhich is at least partially martensitic and preferably fullymartensitic.

The principal constituents of steel which determine its properties areferrite and cementite. At a relatively high temperature, which isdependent upon the carbon content, steel exists in the form known asaustenite which is a solid solution of carbon or cementite in ferrite.When steel is cooled slowly from a high temperature at which austeniteis stable, the ferrite and cementite precipitate together in acharacteristic lamellar structure known as pearlite. However, dependentupon the rate of quenching and other factors, the transformation fromaustenite to pearlite proceeds through a series of differentmicrostructures. The lowtemperature transformation product in thetransformation of austenite upon cooling is martensite which is abody-centered tetragonal structure in which the carbon atoms arethoroughly dispersed. Martensitic steels are characterized by hightensile and yield strengths.

Plain carbon steels of relatively high carbon content and certain alloysteels, particularly those containing hardenability agents such asboron, are more easily quenched to a martensitic structure, but theplain carbon steels of relatively low carbon content are considerablymore difficult to quench to martensite. As will be understood from thecustomary isothermal transformation diagrams, plain carbon steels of lowcarbon content (0.03-0.25 weight percent) require extremely rapidquenching in order to transform substantially all the austenite tomartensite. In general, it may be stated that low carbon steel stripmust be quenched from the austenitizing temperature to below thetemperature for the start of martensite formation in about 0.1 to about0.8 seconds. In addition, the quenching must be accomplished uniformlyso as to obtain a uniform microstructure and so as to avoid excessivewarpage or distortion ofthe strip.

In my prior U.S. Pat. No. 3,410,734, granted Nov. 12, 1968, I havedescribed and claimed a channel quench system which is particularlyadapted for the continuous quenching of lowcarbon steel strip to obtaina fully or partially martensitic microstructure. In such system, it isfound that in most cases a certain amount of in situ tempering orself-tempering of the martensite can occur during the quenching of thestrip because the temperature range of martensite formation isrelatively high for plain carbon steels of low-carbon content (0.03 to0.25 weight percent). Consequently, in spite of the rapidity of thequench provided by such system, a significant degree of tempering cantake place during the short time required to cool the strip from themartensite transformation temperature range to ambient temperature.

However, it is desirable to be able to regulate the extent of temperingof the martensite rather than to be dependent solely upon theuncontrolled self-tempering effect which occurs as described above. Bymodifying the operation of a channel quench system, such as the onedisclosed in my aforementioned patent, it is possible to provideselective control over the heat withdrawal rate in different portions ofthe quench system and thereby achieve a controlled degree of tempering.Thus, within limits, it is possible to tailor the quench action to thechemistry of the steel and other process variables so as to obtainvarying combinations of physical properties in the end product.

Accordingly, the broad object of the invention is to provide a novel andimproved quench system for making low-carbon martensitic steel strip.

A further object of the invention is to provide a novel and improvedmethod for quenching low-carbon steel strip in a channel quench systemso as to obtain at least a partially martensitic microstructure with adesired degree of tempering of the martensite.

Another object of the invention is to provide a novel and improvedquench method for making low-carbon martensitic steel stripcharacterized by the provision of different heat withdrawal rates indifferent portions of the quench system so as to control the extent oftempering of the martensite.

An additional object of the invention is to provide a novel and improvedmethod for increasing the extend of tempering of martensite during thequenching of low-carbon steel strip to form martensitic steel strip.

Other objects and advantages of the invention will become apparent fromthe subsequent detailed description taken in conjunction with theaccompanying drawing, wherein:

H6. 1 is a schematic vertical sectional view of one embodiment of aquench apparatus for the production of martensitic steel strip inaccordance with the present invention; and

FIG. 2 is a similar view of another embodiment of the quench apparatus.

The quench system of the present invention is illustrated in the drawingas embodied in a continuous heat-treating and quenching line for makingmartensitic steel strip. The resulting martensitic steel strip may beused as such or it may be tin plated, galvanized, or aluminum coated.

The steel strip starting material is plain carbon steel having thefollowing composition range (weight percent): carbon 0.03-0.25,manganese 0.20-0.60, phosphorus 0.05 max., sulfur 0.03 max., and thebalance iron with residual elements in the usual amounts. Preferably,the steel strip starting material is in work-hardened or as-cold reducedcondition, and although the gauge of the strip will usually andpreferably be within the range of from about 0.002 to about 0.050 inch,the invention in its broadest aspect is also applicable to steel striphaving a thickness as low as about 0.0002 inch and as high as about0.100 inch.

As seen in FIG. 1, steel strip 10 is fed downwardly through a furnace,shown fragmentarily at 15, after first passing through conventionalcleaning and rinsing steps (not shown) in which the residual rolling oilis removed. In the furnace 15 the steel strip is heated to a uniformtemperature above the A critical point so that the steel is at leastpartially austenitized. This temperature may range from about l,330 F.to as high as about 2, l 00 F., dependent upon the carbon content, butfrom a practical standpoint effective results may be obtained within therange of from about l,330 F. to about 1,750 F. In order to obtain afully martensitic product the steel strip must be heated above the Acritical point, i.e. to a temperature within the range of from about1,525 F. to about 2,l00 F. and particularly within the range of fromabout l,525 F. to about l,750 F. Immediately upon leaving the furnace 15the heated strip passes into the quench system, designated generally at20, where the strip is rapidly quenched to ambient or room temperatureso as to obtain at least a partially martensitic microstructure.Preferably, the strip is quenched at a rate in excess of the criticalcooling rate so that substantially all of the austenite is transformedto martensite.

The quench system 20 comprises a tank 30 provided with a strip exitchute 31 and containing a sinker roll 32. Extending upwardly from thetop wall of the tank 30 is an elongated conduit 34 of rectangular crosssection which provides a restricted quench channel 35. A trough 36having a rectangular cross section surrounds the upper end of theconduit 34. A rectangular baffle 37 depends from an inwardly extendingperipheral flange 33 on the trough 36, the baffle 37 surrounding theconduit 34 and terminating above the bottom of the trough 36. Extendingdownwardly from the outlet end of the furnace 15 is a tubular connectingor seal section 38 the lower end of which extends into the trough 36below the upper end of the conduit 34 and disposed between the conduit34 and the baffle 37. Water (or other quench liquid) is supplied to thetrough 36 by inlet pipes 39, the water flowing downwardly and thenceupwardly around the baffle 37 and then overflowing the open upper end ofthe conduit 34, as indicated by the arrows. The quench water and thestrip l0 pass concurrently downwardly through the channel 35 into thetank 30. The strip 10 passes around the sinker roll 32 and thenoutwardly through the chute 31. The quench water flows from the tank 30upwardly through the chute 31 and is discharged through a side outlet39'.

Since the level of the water overflowing the upper end of the conduit 34is above the lower end of the seal section 38, it will be seen that thesection 38 is sealed by the water in the trough 36 so as to preventinfiltration of air into the furnace 15. if desired, a reducing or othernonoxidizing gas may be supplied to the section 38 (by means not shown)for passage upwardly through the furnace 15, thereby preventingoxidation of the strip. A plurality of view ports 40 are provided in thetubular section 38 and the conduit 34 to permit observation of thequench action.

Submerged jet or spray units 41 are provided in the opposite walls ofthe conduit 34 somewhat below the upper end of the conduit for directingstreams of liquid toward opposite sides of the strip across the entirewidth thereof. In the embodiment shown in FIG. 1, two superimposed units41 are mounted at each side of the strip 10, but any desired number ofsuch units may be used. Each unit 41 is supplied with water or otherquench liquid by means of a supply pipe 42.

As more fully described in my aforementioned prior patent, each sprayunit 41 comprises an elongated boxlike structure having the liquidsupply pipe 42 extending into an opening in its rear wall and having anelongated discharge orifice 43 in its front wall extending substantiallythe entire length of the unit 41 and across the entire width of thestrip 10. The interior of the enclosure is provided with a pair ofelongated oppositely disposed baffles 44 extending between the ends ofthe enclosure and dividing the same into parallel interconnectedchambers. The quench liquid in passing from the inlet 42 to the orifice43 follows a tortuous path in which the direction of fiow is reversedseveral times, thereby insuring a uniform distribution of liquid acrossthe entire length of the orifice opening 43.

As will be evident from the drawing, the submerged spray units 41 aremounted in suitable openings in the: walls of the conduit 34 so thatthin sheets or curtains of water are directed from the orifices 43substantially perpendicularly toward opposite sides of the strip 10. Influid kinematics terminology, the unitary sheet or curtain of water froman elongated rectangular orifice, such as 43, may be characterized ashaving twodimensional flow, i.e. the flow is identical in parallelplanes so as to extend uniformly across the width ofthe strip 10.

As the heated steel strip 10 moves downwardly from the furnace it passesthrough the seal section 38 and enters the upper end of the water-filledquench channel 35 where it is immediately immersed in the downwardlyflowing stream of water. In addition, the submerged spray units 41direct water streams against the strip in a direction generallyperpendicular to the path of movement of the strip, thereby creating ahigh degree of turbulence in the uppermost portion or strip entry end ofthe quench channel. The dimensions of the conduit 34 are restricted soas to provide in the channel 35 a high velocity of water flow relativeto the strip 10 while at the same time allowing sufficient clearance topermit passage of the strip 10 without scraping the walls of theconduit.

As previously mentioned, uniformity of quenching is essential not onlyfor the sake of obtaining a strip having uniform microstructure anduniform physical properties but also to avoid warpage and distortion ofthe strip. lrregular vaporization of the water or other quenching mediumin contact with the strip can result in substantial differentials inheat transfer rates between portions of the strip surface in contactwith liquid water and other portions in contact with water vapor. Thesedifferentials cause different rates of contraction in the steel stripand result in quenching stresses and deformation.

However, in the quench system of the present invention the desireduniformity of quenching is realized as a result of several cooperatingfactors. The provision of the restricted quench channel 35 results in ahigh water velocity relative to the strip 10 which, by way of example,may be on the order of l to 10 feet/second in a direction generallyparallel to the strip 10. Furthermore, the submerged sprays 41 aredesigned so as to provide water streams in a direction generallyperpendicular to the strip 10 at relatively low pressures and relativelyhigh flow rates so as to create substantial turbulence within thechannel adjacent the strip entry end thereof. For example, the waterpressure in the sprays 41 may be on the order of 20 to 30 psi. at theinlets 42 and on the order of5 to 10 p.s.i. at the discharge orifices43. Although the submerged jets 41 are designed to create internalturbulence in the quench channel, nevertheless, the surface of theliquid in the channel 35 where the strip 10 first contacts the quenchliquid is maintained substantially smooth, nonsplashing, andnonturbulent so that every point across the width of the strip makesinitial contact with the quench liquid at substantially the same time.The low pressure of the high flow rate submerged jets 41 makes itpossible to provide the desired surface smoothness while at the sametime providing the required internal turbulence below the liquidsurface. Thus, a quenched strip of at least partially martensiticstructure is obtained which is either flat enough for its intended useor can easily be rolled to flatness.

Any suitable quenching liquid may be used including water, brine orother aqueous salt solution, oil, liquid nitrogen, etc. However, forquenching a heated steel strip to convert austenite to martensite, thepreferred quenching media are water and aqueous brine or other aqueoussalt solutions. For the latter purpose, the volume rate of flow of thequench liquid must be high enough to achieve the cooling rate requiredto transform the austenite to martensite, and the turbulence of thequench liquid relative to the strip, particularly at the strip entry endof the quench channel, must be great enough to prevent the accumulationof vapor film which would lead to nonuniformity of quenching andconsequent distortion of the strip.

Typical line speeds may range from about feet/minute to about 2,000feet/minute dependent upon the gauge of the strip and the carboncontent. The temperature of the quench water is regulated, as describedbelow, and the steel strip will normally be cooled from itsaustenitizing temperature range of l,3302,l00 F. (l,5252,l00 F. in thecase of a fully mar tensitic product) to approximately the watertemperature before leaving the quench tank. If desired, the water orother quench medium may be recirculated through a heat exchanger fortemperature control.

Assuming that the quenching operation has been carried out under optimumconditions, as discussed above, so as to achieve uniformity of quenchingacross the full width of the strip, the final quenched product will haveacceptable flatness for many end uses, as mentioned above. However, thequenched strip can readily be rolled, as on a temper mill, to provideadequate commercial flatness for any desired end use. For example,successful flattening is usually obtained by a single pass through atwin-stand four-high temper mill, each stand having two work rolls andtwo backup rolls. Because of the unusual hardness of a fully martensiticstrip, the work rolls may have a high degree of roughness withoutimpairing the surface of the strip, thereby providing adequateflattening in a single pass.

Other means may also be employed for creating the necessary turbulencein the quench channel 35 by causing highvelocity transverse flow ofcoolant in addition to longitudinal flow of coolant through the channel.For example, the quench channel may be provided with transverse bafflesextending inwardly from the outer walls of the channel toward the stripso as to direct a portion of the quench medium from its generallylongitudinal path parallel with the strip to a transverse path generallyperpendicular to the strip.

The currents or streams of water directed transversely or generallyperpendicularly against the strip by the submerged sprays 41, incooperation with the high relative velocity of the main flow of waterthrough the restricted quench channel 35 parallel to the strip, causesufficient turbulence in the quench medium to eliminate steam pocketswhich would otherwise tend to form or accumulate at the strip surface.Such vapor formation causes a vapor barrier which results in nonuniformquenching, nonuniform transformation to martensite. and consequentwarping and distortion of the strip.

In the quenching action which is accomplished in the channel 35, thequench period may be considered as having three portions. First, thetemperature of the heated strip is lowered rapidly to the temperaturerange of martensite formation, i.e. to the martensite start temperature(M,). Next, the strip is cooled through the temperature range ofmartensite formation, i.e. to the martensite start temperature (M,) tothe martensite finish temperature (M Finally, the strip is cooled toambient temperature or the temperature of the quench liquid. Sincedrastic quenching is required to effect transformation of austenite tomartensite in low-carbon steel, it has generally been thought that thestrip should be quenched as rapidly as possible from its elevatedtemperature through the temperature range of martensite formation. Asheretofore indicated, however, a certain amount of in situ orself-tempering of the martensite takes place in spite of rapid quenchingbecause of the fact that the martensite transformation temperature isquite high for low-carbon steels. For example, for plain carbon steelcontaining 0.03 weight percent carbon and 0.40 weight percent manganese,the martensite start temperature is estimated to be about 990 F. and themartensite finish temperature is estimated as about 610 F. For plaincarbon steel of 0.25 weight percent carbon and 0.40 weight percentmanganese the respective martensite start and finish temperatures areestimated as about 805 F. and about 470 F.

In accordance with the present invention, highly advantageous resultsare obtained by selective control of the rate of quenching or heatwithdrawal in different portions of the quench period. This, in theinitial portion of the quench period the temperature of the strip islowered as rapidly as possible from its elevated austenitizingtemperature to the temperature range of martensite formation, i.e. tothe martensite start temperature. Thereafter, however, the rate ofquenching or heat withdrawal is substantially diminished duringquenching through the temperature range of martensite formation, i.e.from the martensite start temperature to the martensite finishtemperature. As a result of this selective variation or control, anincreased tempering of the martensite is obtained, as compared with aquench system relaying wholly on in situ or selftempering, so as toobtain a final product having improved ductility for a given tensilestrength level. For example, increasing the quench time through thetemperature range of martensite formation from 0.3 second to 0.5 secondcan result in a significant increase in the tempering efiect on themartensitic structure.

Although the invention, in its broadest aspect, is not limited to anyspecific way of obtaining the desired selective control over quench ratein the quench channel, a particularly convenient method comprisesregulating the temperature and flow velocity of the quench liquid in thedifferent portions of the quench channel. More specifically, a portionof the quench liquid is introduced at one end of the elongated quenchchannel and another portion of quench liquid is introduced into thechannel at a submerged location within the channel and adjacent to butspaced from the strip entry end of the channel. By maintaining atemperature differential between the two portions of quench liquid, thedesired control over quench rate can be obtained.

Referring to the quench system shown in FIG. 1 wherein the strip and thequench liquid move concurrently downwardly through the quench channel,it will be seen that the strip is first contacted by the water which isintroduced through the inlets 39 to the trough 36 and overflows theupper end of the conduit 34. Thereafter, the strip 10 is contacted by amixture of the water introduced through the lines 39 and the additionalwater introduced through the submerged spray units 41. The desireddifference in quench rate is obtained by regulating the temperature ofthe separate streams of water supplied through the inlets 39 and 42.Since it is necessary to quench the strip initially as rapidly aspossible to the martensite start temperature, the water supplied throughthe inlets 39 should be at a relatively low temperature inasmuch as thiswater makes the initial contact with the strip 10. Thereafter, in orderto slow down the rate of heat withdrawal as the strip is quenchedthrough the temperature range of martensite formation, the watersupplied to the spray units 41 through the lines 42 should be heated toa relatively higher temperature. The respective temperatures of the twosources of water and the relative volume flow rates from the two sourcesare such that the temperature of the combined or commingled waterstreams is higher than the temperature of the water supplied through theinlets 39, thereby effecting heat withdrawal from the strip at adiminished rate as the strip temperature is lowered through themartensite transformation range.

As previously explained, a plurality of submerged spray units 41 arepreferably provided in superimposed relation in the quench conduit 34 atthe opposite sides of the strip 10, and the temperature of the watersupplied through the spray units at successive elevations may beadjusted as required to insure a diminished rate of heat withdrawal onlyafter the strip temperature has been lowered to the martensite starttemperature. For example, in FIG. 1, it might be desirable to have thetemperature of the water introduced through the uppermost spray units 41substantially the same as the temperature of the water introducedthrough the inlets 39, and higher temperature water would be introducedonly through the lowermost spray units 41. Also, instead of having fixedspray units 41, a similar result could be obtained by providingmechanical means for adjusting the elevations of the spray units 41relative to the quench conduit 34. By either means, however, it ispossible to select the proper elevation for efiecting a diminishedquench rate corresponding to the point at which the martensitetransformation begins. As will be understood, the latter location willvary with strip gauge, line speed, and steel chemistry.

By way of example, for low-carbon steel strip having a thickness of0.024 in., a carbon content of 0.03 to 0.25 weight percent, and amanganese content of 0.20 to 0.60 weight percent, the heat withdrawalrate in the initial portion of the quench period may be from about lXlOto about 3X 10 B.t.u./sq. ft./hr., and the diminished heat withdrawalrate when passing through the martensite transformation temperaturerange may be from about 0.25 l0 to about 0.5 l0" B.t.u./sq. ft./hr. Suchheat withdrawal rates may correspond to a time range of from about 0.35sec. to about L33 sec. for the initial portion of the quench period andfrom about 1.24 sec. to about 3.6 sec. during cooling through themartensite transformation range. Obviously, the exact values will dependon the aforementioned variables. As an example of the temperaturedifferential between the different portions of quench water in the FIG.I embodiment, the relatively low-temperature water introduced throughthe inlet 39 may be from about 35 F. to about F. and the relativelyhigher temperature water introduced at one or more elevations throughthe spray units 41 may be heated to a temperature of from about F. toabout 200 F.

In the FIG. 2 embodiment of the invention, the apparatus is modifiedslightly to accommodate countercurrent movement of the quench waterrelative to the downwardly moving strip. Thus, in FIG. 2 the trough 36surrounding the upper end of the quench conduit 34 has been modified bythe omission of the peripheral flange 33, the depending baffle 37, andthe water inlets 39. Instead, an upright baffle or weir 50 extendsupwardly from the bottom of the trough 36 in spaced relation between theouter walls of the trough and the seal conduit 38. The upper end of thebaffle 50 extends above the lower end of the seal conduit 38. Oneportion of quench water is introduced to the system through an inlet 51pipe extending into the tank 30 and passes upwardly through the quenchchannel 35 from the lower end thereof. As the water approaches the upperend of the quench channel 35 it is commingled with additional quenchwater introduced through the spray units 4|, and the commingled orcombined water stream overflows the upper end of the quench conduit 34into the trough 36. The water then overflows the upper edge of thebaffle or weir 50 and is discharged from the trough 36 through an outletpipe 52.

In this embodiment of the invention the portion of the quench waterintroduced through the inlet pipe 51 is heated to a relatively highertemperature, whereas the water introduced through the spray units 41 isat a relatively lower temperature. As a result, the mixture of quenchwater streams which first contacts the downwardly moving strip has alower temperature than the temperature of the quench water at a lowerpoint in the channel 35 below the spray units 41, and the desireddifference in heat withdrawal rates is maintained in the mannerpreviously described. As an example of the temperature differential, theheated water introduced through the line 51 may have a temperature offrom about 120 F. to about 200 F., and the lower temperature waterintroduced through the lines 42 may be from about 35 F. to about 90 F.

In both systems of FIGS. 1 and 2, the variation in heat withdrawal ratein the different portions of the quench channel is obtained byregulating the temperature and flow velocity of the quench water. ln theconcurrent flow system of HO. 1, the maximum flow rate and liquidvelocity in the channel 35 is below the spray units 41, and thetemperature of the water at this point is greater than the temperatureof the water in the channel above the spray units 41 so that the netresult is a lower rate of heat withdrawal in the lower portion of thequench channel. ln the countercurrent flow system of FIG. 2, thetemperature of the water in the channel 35 below the spray units 41 isagain higher than the temperature of the water above the spray units 41,but, in this case, the waterflow rate and liquid velocity is greater inthe upper portion of the quench channel above the spray units 41. Thus,the net result in the HQ. 2 system is that a higher heat withdrawal rateis obtained in the upper portion of the quench channel.

Although the invention has been described with reference to certainspecific embodiments, it should be understood that various modificationsand equivalents may be used without departing from the scope of theinvention as defined in the appended claims. In addition, although theinvention has been described with reference to the continuous quenchingof steel strip, it is also within the scope of the invention to utilizeseparate sheets of steel.

lclaim:

l. in the method of quenching a strip of plain carbon steel having acarbon content of from about 0.03 weight percent to about 0.25 weightpercent and a manganese content of from about 0.20 weight percent toabout 0.60 weight percent wherein the strip is heated to a temperatureabove the A critical point so as to at least partially austenitize thesteel and thereafter the heated strip is passed through an elongatedrestricted quench channel and quench liquid is also passed through saidchannel for quenching the strip at a rate in excess of the criticalcooling rate so as to transform substantially all of the austenite tomartensite; the improvement which comprises controlling the quenchconditions in one portion of said channel adjacent the strip entry endthereof so as to obtain a relatively high heat withdrawal rate duringthe initial portion of the quench period and thereby rapidly loweringthe strip temperature to the range of martensite formation, andcontrolling the quench conditions in the remaining portion of saidchannel so as to obtain a relatively lower heat withdrawl rate duringquenching through the temperature range of martensite formation, therebyeffecting tempering of the martensite.

2. The method of claim 1 further characterized in that said quenchconditions are controlled by regulating the temperature and flowvelocity of the quench liquid in the respective portions of the quenchchannel.

3. The method of claim 1 further characterized in that the heatwithdrawal rate in said one portion of said channel is from about i X10to about 3X10 B.t.v./sq. ft./hr. and the heat withdrawal rate in saidremaining portion of said channel is from about 0.25Xl0" to about 0.5Xl0B.t.u./sq. ft./hr.

4. The method of claim 1 further characterized in that the time forlowering the temperature of the heated strip to the range of martensiteformation is from about 0.35 sec. to about 1.33 sec., and the time forquenching the strip through the temperature range of martensiteformation is from about L24 sec. to about 3.6 sec.

5. The method of claim 1 further characterized in that the quench liquidpassed through said one portion of said channel is at a lowertemperature than the quench liquid passed through said remaining portionof said channel.

6. The method of claim 5 further characterized in that said lowertemperature is from about 35 F. to about F. and the quench liquid passedthrough said remaining portion of said channel is at a temperature offrom about F. to about 200 F.

7. The method of claim 1 further characterized in that said channel isdisposed vertically and said strip and said quench liquid are passeddownwardly through said channel in concurrent relation.

8. The method of claim 1 further characterized in that said channel isdisposed vertically and said strip is passed downwardly through saidchannel and said quench liquid is passed upwardly through said channelin countercurrent relation.

9. The method of claim 1 further characterized in that one portion ofsaid quench liquid is introduced into one end of said channel andanother portion of said quench liquid is introduced into said channel ata submerged location within said channel adjacent to but spaced from thestrip entry end of the channel.

10. The method of claim 9 further characterized in that said otherportion of said quench liquid is directed in the form of liquid sheetssubstantially perpendicularly against the opposite surfaces of the stripand extending uniformly across the width of the strip.

11. The method of claim 9 further characterized in that said strip andsaid quench liquid are passed through said channel in concurrentrelation, and said other portion of said quench liquid has a highertemperature than said one portion of said quench liquid.

12. The method of claim ll further characterized in that said quenchliquid comprises an aqueous liquid, said one portion of said quenchliquid being at a temperature of from about 35 F. to about 90 F. andsaid other portion of said quench liquid being at a higher temperaturewithin the range offrom about 120 F. to about 200 F.

13. The method of claim 9 further characterized in that said strip andsaid quench liquid are passed through said channel in countercurrentrelation, and said one portion of said quench liquid has a highertemperature than said other portion of said quench liquid.

14. The method of claim 13 further characterized in that said quenchliquid comprises an aqueous liquid, said other portion of said quenchliquid being at a temperature of from about 35 F. to about 90 F. andsaid one portion of said quench liquid being at a higher temperaturewithin the range of from about i 20 F. to about 200 F.

15. in the method of quenching a strip of plain carbon steel having acarbon content of from about 0.03 weight percent to about 0.25 weightpercent and a manganese content of from about 0.20 weight percent toabout 0.60 weight percent wherein the strip is heated to a temperatureabout the A, critical point so as to at least partially austenitize thesteel and thereafter the heated strip is passed through an elongatedrestricted quench channel and quench liquid is also passed through saidchannel for quenching the strip at a rate in excess of the criticalcooling rate so as to transform substantially all of the austenite tomartensite, one portion of said quench liquid being introduced into oneend of said channel and another portion of said quench liquid beingintroduced into said channel at a submerged location within said channeladjacent to but spaced from the strip entry end of the channel; theimprovement which comprises:

regulating the temperature and flow velocity of the quench liquid in oneportion of said channel adjacent the strip entry and thereof so as toobtain during the initial portion of the quench period a relatively highheat withdrawal rate which is sufficient to rapidly lower the striptemperature to the martensite start temperature; and

regulating the temperature and flow velocity of the quench liquid in theremaining portion of said channel so as to obtain, during quenching fromthe martensite start temperature to the martensite finish temperature, aheat withdrawal rate which is less than the heat withdrawal rate in saidone portion of said channel, thereby effecting increased tempering ofthe martensite.

16. The method of claim 15 further characterized in that said strip andsaid quench liquid are passed through said channel in concurrentrelation, and said other portion of said quench liquid has a highertemperature than said one portion of said quench liquid.

17. The method of claim 16 further characterized in that said quenchliquid comprises an aqueous liquid, said one portion of said quenchliquid being at a temperature of about 35 F. to about F. and said otherportion of said quench liquid being at a higher temperature within therange of from about F. to about 200 F.

V 18. The method of claim 15 further characterized is that said stripand said quench liquid are passed through said channel in countercurrentrelation, and said one portion of said quench liquid has a highertemperature than said other portion of said quench liquid.

19. The method of claim 18 further characterized in that said quenchliquid comprises an aqueous liquid, said other portion of said quenchliquid being at a temperature of from about 35 F. to about 90 F. andsaid one portion of said quench liquid being at a higher temperaturewithin the range of from about 120 F. to about 200 F.

2. The method of claim 1 further characterized in that said quenchconditions are controlled by regulating the temperature and flowvelocity of the quench liquid in the respective portions of the quenchchannel.
 3. The method of claim 1 further characterized in that the heatwithdrawal rate in said one portion of said channel is from about 1 X106 to about 3 X 106 B.t.v./sg. ft./hr. and the heat withdrawal rate insaid remaining portion of said channel is from about 0.25 X 106 to about0.5 X 106 B.t.u./sq. ft./hr.
 4. The method of claim 1 furthercharacterized in that the time for lowering the temperature of theheated strip to the range of martensite formation is from about 0.35sec. to about 1.33 sec., and the time for quenching the strip throughthe temperature range of martensite formation is from about 1.24 sec. toabout 3.6 sec.
 5. The method of claim 1 further characterized in thatthe quench liquid passed through said one portion of said channel is ata lower temperature than the quench liquid passed through said remainingportion of said channel.
 6. The method of claim 5 further characterizedin that said lower temperature is from about 35* F. to about 90* F. andthe quench liquid passed through said remaining portion of said channelis at a temperature of from about 120* F. to about 200* F.
 7. The methodof claim 1 further characterized in that said channel is disposedvertically and said strip and said quench liquid are passed downwardlythrough said channel in concurrent relation.
 8. The method of claim 1further characterized in that said channel is disposed vertically andsaid strip is passed downwardly through said channel and said quenchliquid is passed upwardly through said channel in countercurrentrelation.
 9. The method of claim 1 further characterized in that oneportion of said quench liquid is introduced into one end of said channeland another portion of said quench liquid is introduced into saidchannel at a submerged location within said channel adjacent to butspaced from the strip entry end of the channel.
 10. The method of claim9 further characterized in that said other portion of said quench liquidis directed in the form of liquid sheets substantially perpendicularlyagainst the opposite surfaces of the strip and extending uniformlyacross the width of the strip.
 11. The method of claim 9 furthercharacterized in that said strip and said quench liquid are passedthrough said channel in concurrent relation, and said other portion ofsaid quench liquid has a higher temperature than said one portion ofsaid quench liquid.
 12. The method of claim 11 further characterized inthat said quench liquid comprises an aqueous liquid, said one portion ofsaid quench liquid being at a temperature of from about 35* F. to about90* F. and said other portion of said quench liquid being at a highertemperature within the range of from about 120* F. to about 200* F. 13.The method of claim 9 further characterized in that said strip and saidquench liquid are passed through said channel in countercurrentrelation, and said one portion of said quench liquid has a highertemperature than said other portion of said quench liquid.
 14. Themethod of claim 13 further characterized in that said quench liquidcomprises an aqueous liquid, said other portion of said quench liquidbeing at a temperature of from about 35* F. to about 90* F. and said oneportion of said quench liquid being at a higher temperature within therange of from about 120* F. to about 200* F.
 15. In the method ofquenching a strip of plain carbon steel having a carbon content of fromabout 0.03 weight percent to about 0.25 weight percent and a manganesecontent of from about 0.20 weight percent to about 0.60 weight percentwherein the strip is heated to a temperature above the A1 critical pointso as to at least partially austenitize the steel and thereafter theheated strip is passed through an elongated restricted quench channeland quench liquid is also passed through said channel for quenching thestrip at a rate in excess of the critical cooling rate so as totransform substantially all of the austenite to martensite, one portionof said quench liquid being introduced into one end of said channel andanother portion of said quench liquid being introduced into said channelat a submerged location within said channel adjacent to but spaced fromthe strip entry end of the channel; the improvement which comprises:regulating the temperature and flow velocity of the quEnch liquid in oneportion of said channel adjacent the strip entry and thereof so as toobtain during the initial portion of the quench period a relatively highheat withdrawal rate which is sufficient to rapidly lower the striptemperature to the martensite start temperature; and regulating thetemperature and flow velocity of the quench liquid in the remainingportion of said channel so as to obtain, during quenching from themartensite start temperature to the martensite finish temperature, aheat withdrawal rate which is less than the heat withdrawal rate in saidone portion of said channel, thereby effecting increased tempering ofthe martensite.
 16. The method of claim 15 further characterized in thatsaid strip and said quench liquid are passed through said channel inconcurrent relation, and said other portion of said quench liquid has ahigher temperature than said one portion of said quench liquid.
 17. Themethod of claim 16 further characterized in that said quench liquidcomprises an aqueous liquid, said one portion of said quench liquidbeing at a temperature of about 35* F. to about 90* F. and said otherportion of said quench liquid being at a higher temperature within therange of from about 120* F. to about 200* F.
 18. The method of claim 15further characterized is that said strip and said quench liquid arepassed through said channel in countercurrent relation, and said oneportion of said quench liquid has a higher temperature than said otherportion of said quench liquid.
 19. The method of claim 18 furthercharacterized in that said quench liquid comprises an aqueous liquid,said other portion of said quench liquid being at a temperature of fromabout 35* F. to about 90* F. and said one portion of said quench liquidbeing at a higher temperature within the range of from about 120* F. toabout 200* F.